The present invention relates to a single-layer type electrophotosensitive material which is used in digital image forming apparatuses such as electrophotosensitive copying machine, facsimile and laser beam printer, and a digital image forming method using the same.
More particularly, the present invention relates to a single-layer type electrophotosensitive material, which does not generate a memory image even when using in a reversal development type digital image forming apparatus including no charge neutralizing step, and a reversal development type digital image forming method using the same, which does not include a charge neutralizing step.
Recently, an organic photosensitive material has widely been used because of its easy production, low cost, wide range of choice of photosensitive materials such as electric charge transferring material, electric charge generating material and binder resin, and high functional design freedom as compared with a conventional inorganic photosensitive material.
The organic photosensitive material includes, for example, single-layer type photosensitive material wherein an electric charge transferring material (hole transferring material, electron transferring material) is dispersed in the same photosensitive layer, together with an electric charge generating material, and multi-layer type photosensitive material comprising an electric charge generating layer containing an electric charge generating material and an electric charge transferring layer containing an electric charge transferring material, which are mutually laminated. The single-layer type photosensitive material has attracted special interest recently because it has such an advantage that optical characteristics can be improved due to simple layer structure, excellent productivity and less interfaces between layers.
On the other hand, an image forming apparatus using an electrophotosensitive system is capable of charging a photosensitive material (principal charge step), exposing an image to form a static latent image (exposure step), developing the static latent image with a toner at a state where a developing bias voltage is applied (development step) transferring the formed toner image to a transfer paper (transfer step), and fixing to form an image. The residual toner on the photosensitive material is cleaned by a urethane blade (cleaning step) and the residual electric charges on the photosensitive material are neutralized by LED (charge neutralizing step).
To reduce the size of the image forming apparatus and initial cost, various trials of omitting the cleaning step and charge neutralizing step have been made.
The image forming apparatus using an electrophotosensitive system includes, for example, digital and analogue copying machines, facsimile and laser beam printer. In particular, a reversal development system for developing using a toner having the same polarity as that of a charging voltage to be applied to the photosensitive material in the charge step has widely been used in a digital image forming apparatus.
However, the following problems such as generation of a memory image occur when using a conventional electrophotosensitive material in a reversal development type digital image forming apparatus.
<Transfer memory>
When using an electrophotosensitive material in a reversal development type digital image forming apparatus, a transfer voltage to be applied to the electrophotosensitive material in a transfer step is usually applied through a transfer medium (paper) without being applied directly to the electrophotosensitive material, and the transfer voltage is not applied when the transfer medium does not pass through the transfer step.
However, on-off timing of the transfer voltage is very difficult and portion to be applied directly to the photosensitive material is often generated with respect to front/rear portions of the transfer medium. That is, application of the transfer voltage starts before a transferring apparatus is covered with the front portion of the transfer medium. Furthermore, the transfer voltage is continuously applied even if portion of the transferring apparatus is exposed by passage of the rear end of the transfer medium so that the transfer voltage is applied directly to the photosensitive material at said portion.
In case of a positively charging single-layer type photosensitive material, since the polarity of the voltage to be applied in the transferring apparatus is negative, negative space electric charges are remained at the portion of the photosensitive material to which a negative voltage has been applied. Generally, a single-layer type photosensitive material has sensitivity in both polarities so that negative space electric charges are neutralized in the following charge neutralizing step.
However, in case where the sensitivity of the positively charging single-layer type photosensitive material to the negative polarity is drastically inferior (mobility of the electron transferring material is very small) or the photosensitive material is used in the image forming apparatus including no charge neutralizing step, negative space electric charges are not sufficiently neutralized and a reduction in potential is caused by an influence of space electric charges even if the photosensitive material is positively charged in the following charge step. Furthermore, a difference in sensitivity appears in the development step, thus causing such problems that said portion turns into black in the image (memory image).
<Exposure memory>
After passing through the exposure step and development step, positive electric charges on the surface of the positively charging single-layer type photosensitive material are uniformly neutralized in the charge neutralizing step usually, and the photosensitive material is positively charged uniformly in the following charge step.
Similar to the case of the transfer memory, the negative space electric charge density of the exposed portion is larger than that of the non-exposed portion and a difference in potential appears in the following charge step and the memory image is liable to be generated in case where the sensitivity of the positively charging single-layer type photosensitive material to the negative polarity is inferior and the photosensitive layer is used in the image forming apparatus including no charge neutralizing step.